The spatial coherence of the seismograms observed at a narrow-aperture array on hard rock sites in the Adirondack Mountains is approximately exp(- cf-DELTA-x), where c about 0.67 km-1 Hz-1, f is frequency and DELTA-x is the distance between array elements. Vertical, radial, and transverse components were quite coherent over the aperture of the array (> 100 m), indicating that the transverse motion of the compressional wave is a property of relatively large (10(6) m3) volumes of rock, and not just an anomaly caused by a malfunctioning instrument, poor instrument-rock coupling, or outcrop-scale effects. The spatial coherence is approximately independent of component, epicentral azimuth and range, and whether P- or S-wave coda is being considered, at least for propagation distances between 5 and 170 km. The travel time of the seismic waves can be modelled by a simple, three layer model, but such a model produces an unacceptably small amount of coda. The introduction of thin vertically stratified velocity heterogeneities leads to significant coda, but the scale length of spatial coherence of this coda is an order of magnitude greater than is observed. A model of scattering from an ensemble of point scatterers randomly placed in the shallow crust is capable for reproducing the observed coherence data, including its functional dependence on the quantity, f-DELTA-x.
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